Magnetic stirring system

ABSTRACT

A magnetic stirrer wherein a relatively weakly magnetised disposable stirrer bar inside a closed container, for example a stirrer bar moulded wholly of non-isotropic ferrite, is moved with a stirring action from outside the container by a relatively powerful magnetic driver, preferably consisting of a pair of permanent magnets of neodymium boron driven in rotation as an assembly.

FIELD OF THE INVENTION

This invention relates generally to a magnetic stirring system and inparticular to a magnetic stirrer and a method of magnetically stirring aliquid within a closed culture bottle.

BACKGROUND TO THE INVENTION

Magnetic stirring of liquids within a closed bottle is a well knowntechnique. A stirrer bar in the liquid to be stirred is moved with astirring action by a magnetic field producing means outside the bottle.The magnetic field producing means outside the bottle may be motordriven magnets or solid state switched coils. The stirrer bar normallyused hitherto in such systems is of relatively high quality, beingstrongly magnetised and costly to manufacture. Accordingly, it is normalto recover the stirrer bar after use and wash it for further use.

Known from German Specification No. 1 757 099 is a magnetic stirringmeans for a bank of test tubes. Stirrer bars of unquantifiedmagnetisation are driven in unspecified motion by an external permanentmagnetic means driven in rotation. Also known from U.S. Pat. No.4,090,263 is a magnetic stirrer system in which a magnetisable stirrerbar is coupled to an external drive magnet. It is well understood,however, that in a closed culture bottle an essential aim of any stirrersystem is to promote gas exchange with the bottle headspace bydisturbing the liquid surface. It is not apparent that the known stirrersystems are able to achieve this aim.

However, there are applications for magnetic stirrers, such as thestirring of cultures in microbiology, where the fluid is often extremelyhazardous and recovery of stirrer bars would require protectedsterilisation. In such circumstances, removal of the stirrer bar fromthe culture bottle would be a very undesirable process. Furthermore,culture bottles are themselves a disposable product, and must be ofcheap manufacture. The incorporation of expensive stirrer bars in suchbottles has therefore not been considered a practicable procedure, sothat magnetic stirring has not been considered possible in this field ofapplication.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a magneticstirrer comprising a bottle unit constituted by a bottle having a baseand a stirrer bar of relatively low power magnetisation lying on thebottle base within the bottle, and a permanent magnet means ofrelatively high power located beneath the bottle base in close proximitythereto, and driving means for continuously rotating the externalpermanent magnet means about an axis substantially normal to the bottlebase so that its rotating strong magnetic field entrains the stirrer barin continuous rotation in a plane parallel to and above the bottle basewithin the liquid in the container.

According to another aspect of the invention, there is provided a methodof magnetically stirring a liquid within a bottle, wherein a stirrer barof relatively low power magnetisation is inserted into the bottle to lieon the bottle base, a permanent magnet means of relatively high power iscontinuously driven in rotation beneath the bottle base in closeproximity thereto and about an axis substantially normal to the bottlebase, causing its rotating strong magnetic field to entrain the stirrerbar in continuous rotation in a plane parallel to and above the bottlebase within the liquid in the container.

The invention is based on the realisation that an infallible magneticcoupling between an internal stirrer bar and an external magnetic drivercan be achieved, with a low cost stirrer bar of low magnetisation,provided that the necessary coupling can be ensured by use of a highpower magnetic drive.

In the field of blood culture bottles especially, a reliable magneticcoupling between the stirrer bar and its magnetic driving means is anessential requirement, because the bottle containing the sample iseffectively opaque, so that visible checking of the operation of thestirrer bar is virtually impossible.

Because the stirrer bar is of relatively low power, it can besufficiently cheaply made to be disposable with the liquid container.Preferably, therefore, the stirrer bar is moulded wholly of ferrite innon-isotropic form. A more powerfully magnetised bar using isotropicferrite can be produced, but manufacture involves orientating theferrite during moulding, and this is a costly procedure Such a morepowerfully magnetised stirrer bar could therefore not be treated asdisposable in the context of the market relating to culture bottles.

For the low power stirrer bar to be reliably driven with a stirringaction, the external magnetic drive must be of such compensatory highpower as to ensure a reliable magnetic coupling. This is preferablyachieved by use of a pair of spaced high power magnets respectively withnorth and south poles adjacent the stirrer bar. Rare earth magnets arepreferred, in particular of neodymium boron.

The stirrer bar is preferably magnetised along its longitudinal axis.The external permanent magnets, more especially of neodymium boron, thenpreferably extend generally normally to the base of the container withspaced centres, and the driving means acts to rotate the magnets aboutan axis parallel to the lengths of the magnets and generally centralbetween said magnets.

A practical magnetic stirrer and method of magnetic stirring inaccordance with the invention will now be described by way of examplewith reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a diagrammatic view of a magnetic stirrer inaccordance with the present invention.

Referring to drawing, a container 10, for example a bottle containing aculture 12, for example a blood culture, has placed within it aninexpensive stirrer bar 14 consisting of a weak magnet moulded wholly offerrite in non-isotropic form. Because the stirrer bar 14 is cheaplymanufactured, it is disposable with the culture bottle 12 after use.

In use, the stirrer bar 14 is moved with a stirring action within theblood culture 12 by an external magnetic driver generally referenced 16.In order to compensate for the weak magnetic power of the stirrer bar14, the magnetic driver 16 must be of very high power, and consists oftwo permanent magnets 18, 20 of neodymium boron, arranged in spacedparallel relationship one with its north pole and one with its southpole adjacent the stirrer bar. These strong permanent magnets 18, 20 aredriven in rotation as an assembly about an axis 22.

Even stronger coupling with the stirrer bar 14 can be achieved if thepermanent magnets 18, 20 are linked between poles by magnetisablematerial, as indicated in broken line at 24. The arrangement in effectforcibly couples with the stirrer bar 14, reliably drawing it intoalignment with the permanent magnets 18, 20 and maintaining thisalignment as the permanent magnets rotate. The extreme power of thepermanent magnets 18, 20 dominates the coupling action.

Typically, the stirrer bar is a 20 mm long cylindrical component of 6 mmdiameter, magnetised along its longitudinal axis, and the permanentmagnets 18, 20 are each of 6 mm length and 4 mm diameter, being spacedat about 18 mm centres. In the illustrated example, the coupling actionis achieved through the base of the bottle 10.

Although primarily described with reference to use in the field ofstirring cultures, the invention is also useful in other fields, notonly involving other hazardous liquids to be stirred which may makerecovery of the stirrer bar undesirable, but as an alternative, forexample, to the complex engineering to be found in shaker/incubatorsextensively used in microbiological laboratories.

I claim:
 1. A magnetic stirrer comprising a bottle unit constituted by aclosed culture bottle containing liquid and a gaseous headspace abovethe liquid, the bottle having a base and a stirrer bar of relatively lowpower magnetisation establishing opposite poles at opposite ends of thebar and extending parallel to the bottle base within the bottle, and apermanent magnet means of relatively high power providing spacedopposite poles in a plane parallel to the length of the stirrer barlocated beneath the bottle base in close proximity thereto, and drivingmeans for continuously rotating the permanent magnet means about an axissubstantially normal to the bottle base so that its rotating strongmagnetic field aligns with that of the stirrer bar and entrains thestirrer bar in continuous rotation in a plane parallel to and above thebottle base within the liquid in the container, thereby to disturb theliquid surface in order to promote gas exchange with the bottleheadspace.
 2. A stirrer as claimed in claim 1, wherein the stirrer baris a low cost disposable magnet.
 3. A stirrer as claimed in claim 2,wherein the stirrer bar is moulded wholly of ferrite in non-isotropicform.
 4. A stirrer as claimed in claim 1, wherein the permanent magnetmeans comprises a pair of spaced, relatively high power magnetsrespectively with north and south poles adjacent the stirrer bar.
 5. Astirrer as claimed in claim 4 wherein the permanent magnets extendgenerally normally to the base of the bottle with spaced centres, andthe driving means acts to rotate the magnets about an axis parallel tothe lengths of the magnets and generally central between said magnets.6. A stirrer as claimed in claim 4 or 5, wherein the permanent magnetmeans comprises at least one relatively powerful rare earth magnet.
 7. Astirrer as claimed in claim 6, wherein the at least one relativelypowerful magnet is made of neodymium boron.
 8. A method of magneticallystirring a liquid within a closed culture bottle which contains agaseous headspace above the liquid, wherein a stirrer bar of relativelylow power magnetisation establishing opposite poles at opposite ends ofthe bar is inserted into the bottle to extend parallel to the bottlebase, a permanent magnet means of relatively high power providing spacedopposite poles in a plane parallel to the length of the stirrer bar iscontinuously driven in rotation beneath the bottle base in closeproximity thereto and about an axis substantially normal to the bottlebase, causing its rotating strong magnetic field to align with that ofthe stirrer bar and thereby entrain the stirrer bar in continuousrotation in a plane parallel to and above the bottle base within theliquid in the container, thereby to disturb the liquid surface in orderto promote gas exchange with the bottle head space.
 9. A method asclaimed in claim 8, applied to the stirring of a hazardous liquid in adisposable bottle.